In comparison to conventional fixed wing aircraft, helicopters exhibit several advantages. For example, the ability to carry out a vertical take-off and landing makes it possible to operate and maneuver a helicopter in areas having very limited or confined space. Moreover, the ability to fly at a very low forward ground speed makes the helicopter suitable for use in various surveillance operations, and the ability to hover makes the helicopter the principally suitable aircraft for use in search and rescue operations.
Both the vertical lift and the forward thrust for achieving the climb, hovering, and forward flight of the helicopter are generated by the rotation about a rotor mast, of the aerodynamically contoured airflow profile members, which are generally called airfoil members herein, and which are particularly called rotor blades in the context of a helicopter. The rotor blades each generate a lift component that can be oriented depending on the respective position of the rotor blade relative to the rotor mast. A controlled adjustment of these positions of the rotor blades, and particularly the pitch angle positions of the rotor blades, and the resulting control of the lift and forward thrust of the helicopter is normally achieved by tilting or pivoting each rotor blade along a pitch axis extending in the span width direction of the respective rotor blade.
In one type of pitch control arrangement, a variably tiltable swashplate or pitch control cam disk achieves the desired pitch control pivoting of the rotor blades via connecting rods or the like extending between the swashplate and the rotor blades. As an alternative, in a second type of pitch control arrangement, the pitch angle position of each rotor blade is influenced by a respective servo-actuated flap provided along a trailing edge of the respective rotor blade. In this context, a tilting displacement of the servo-flap causes a change in the aerodynamic flow characteristic of the air flowing over the rotor blade, which in turn causes the rotor blade to pivot about its respective span direction axis so as to change the pitch angle of the rotor blade.
It has also been found that servo-flaps can be used in order to reduce the so-called "blade slap" or slapping noise that is typically caused by the rotating rotor blades due to the alternating effect of each rotor blade interacting with air vortices or flow turbulences generated by and separating from the preceding rotor blade. Used in this context, the servo-flaps are actuated so as to reduce the aerodynamic alternating effects such that the air vortices are weakened and pushed further outward from the rotor blade by a slight pitching and flattening or pulling-in of the rotor blade.
Several prior art references disclose the use of servo-actuated flaps in a rotor blade. U.S. Pat. No. 5,588,800 generally discloses the use of a servo-flap for suppressing the blade slap noise of a rotor blade. U.S. Pat. No. 5,387,083 discloses a servo-flap provided on a rotor blade, wherein the servo-flap is driven by a rotational actuator arranged in the rotor blade. U.S. Pat. No. 5,639,215 discloses a servo-flap arrangement for a rotor blade, wherein the servo-flap is driven by an actuator arranged in the rotor blade either directly or via a linkage mechanism.
The most recent investigations and experiments have also shown that servo-flaps can be provided along the trailing edges of wings of conventional non-rotor aircraft, whereby the servo-flaps have a similar influence on the aerodynamic characteristics as a kink-free curving or cambering of the trailing edge. Thus, the general aerodynamically contoured member or airfoil member outfitted with servo-flaps can also be used as a wing of a conventional non-rotor aircraft.
Common to all of the known arrangements of servo-flaps on airfoil members, either electric motors or hydraulic systems are used as actuators for tilting or deflecting the flaps in a direction toward the suction or vacuum side of the airfoil and in a direction toward the pressure side of the airfoil. Both electric motors and hydraulic systems suffer serious disadvantages as actuators for such servo-flaps, especially in the context of servo-flaps provided on rotor blades of a helicopter. For example, electric motors used as actuators are disadvantageously influenced by the high accelerations (up to 1000 G) that arise in the rotor blade during operation. In this context, the accelerations especially arise from the effective centrifugal forces. Such high accelerations and corresponding forces lead to the tilting or jamming displacement of the various individual rotatable components of the electric motors, which are necessarily arranged with a certain degree of bearing play. For this reason, the operation of such electric motors under high acceleration-force conditions is seriously limited.
On the other hand, the use of hydraulic systems as actuators easily causes the airfoil member to exceed its maximum allowable weight, in view of the relatively high weight of the hydraulic system components. Moreover, the actuating speeds of both electric motors and hydraulic systems are too low to be advantageously used as actuators for trailing edge servo-flaps of air foil members.